As communication becomes more important, there is a stronger demand that a high-quality stable communication path be offered. With this background, a system in which a communication path has redundancy is generally adopted. In addition, it becomes important to properly monitor for errors in the communication path and eliminate the errors as much as possible.
Recently, particularly with respect to a transmission network, the SDH (Synchronous Digital Hierarchy) interface is mainly used in a synchronous network. In North America, the SDH interface is adopted as SONET (Synchronous Optical Network). In SDH, a cross connecting operation and a switching operation are performed for an individual path.
FIG. 1 shows, for instance, an STM-1 (Synchronous Transport Module Level 1) interface (155 Mbps) in SDH. In an STM-1 interface, one AU4 (Administrative Unit) or three AU3s can be accommodated therein. FIG. 1 illustrates the case where three AU3s are accommodated in the STM-1 interface. An AU3 is formed by adding AUPTR (AU Pointer) to VC3 (Virtual Container 3)+FS (Fixed Staffbyte), and the three AU3s are byte-interleaved-multiplexed. In FIG. 1, dotted X s in SOH (Section OverHead) represent that the bytes are not used.
In FIG. 1, AUPTR that is formed of H1, H2 and H3 bytes in SOH indicates individual frames of VC3+FS, in this case, relative phases (positions) with respect to SOH. In detail, AUPTR indicates positions of J1 bytes in POHs (Pass OverHead). In addition, AUPTR uses B3 bytes in POHs to monitor a path error.
At a path terminal point in a sender side, for each frame of VC3(+FC)s, BIP (Bit Interleaved Parity) is computed in operation ranges as shown in dotted areas in FIGS. 2(A) and 2(B) and then the operation result is inserted into B3 byte in POH in the next frame. Here, FIG. 2(A) illustrates a case of VC3 of SDH, and FIG. 2(B) illustrates a case of STS-1 of SONET.
At a path terminal point in a receiver side, BIP is computed in a manner similar to the sender side. By comparing the operation result with the received B3 byte, each communication path is monitored for communication path errors. In BIP operation, the operation ranges in FIGS. 2(A) and 2(B) are partitioned by a unit of byte (8 bits) and an even parity operation result of the last bit b0 in each byte is inserted into the last bit b0 in B3 byte. A similar operation is also performed for the other bits b1 through b7. Also, the insertion in B3 byte is performed only for transmission apparatuses located at path terminal points. The insertion in B3 byte is not performed for a transmission apparatus that carries out a path cross-connecting operation and a path switching operation halfway through a communication path.
There are a large number of parts for performing a parallel process of 2n (generally 8) bits with respect to signal processing in a transmission apparatus. As a first problem, when these parts for performing the parallel process have trouble such that, for instance, a part for processing a bit fixes the bit as “0” or “1” due to a short circuit and so on, it is impossible to detect such an error in a conventional BIP operation.
In a network where transmission apparatuses A, B, C and D are connected as shown in FIG. 3, a redundant communication path such as BLSR (Bidirectional Line Switched Ring) is provided between the transmission apparatuses B and C. Here, there are path terminal points in the transmission apparatuses A and D, and the transmission apparatuses B and C between the path terminal points serve to monitor the B3 byte and switch the communication path when a path error is detected therein. When a path error occurs between the transmission apparatuses A and B, the transmission apparatus C cannot restore communication between the transmission apparatuses A and B by switching the path. Thus, in this case, the transmission apparatus C basically does not have to switch the path. However, since delay difference actually arises in the redundant communication paths between the transmission apparatuses B and C, time lag is caused with respect to the path error detection in the transmission apparatus C. In this case, if a path error is detected in a currently used communication path, there arises an unnecessary path switching operation in that the communication path is switched to an alternative communication path although an error is not detected at this time. As a second problem, although this path switching operation is not necessary, an administrator is informed that the system has trouble.